Process for the preparation of the 14β-hydroxy-baccatin III-1,14-carbonate

ABSTRACT

A process for the preparation of 14β-hydroxy-baccatin III-1,14-carbonate useful for the preparation of novel taxane derivatives with antitumor activity.

The present invention relates to a process for the preparation of14β-hydroxy-1,14-carbonate-baccatin III. The product obtained with theprocess of the invention can be used in the preparation of novel taxanederivatives with antitumor activity.

Taxanes are one of the most important classes of antitumor agentsdeveloped in recent years. Paclitaxel is a diterpene complex obtainedfrom the bark of Taxus brevifolia and is considered one of the majormedicaments for the therapy of cancer. At present, an extensive searchis being carried out for novel taxane derivatives having superiorpharmacological activity and improved pharmacokinetic profile. Aspecific approach relates to baccatin III derivatives variously modifiedwith respect to the parent structure. Examples of said compounds arerepresented by the 14β-hydroxy baccatin III derivatives disclosed inU.S. Pat. No. 5,705,508, WO 97/43291, WO 96/36622. At present,14β-hydroxy-deacetylbaccatin III 1,14-carbonate derivatives are preparedstarting from the precursor 14β-hydroxy-deacetylbaccatin III, which is anatural compound obtainable in small amounts by extraction of the leavesof Taxus wallichiania, as disclosed in EP 559 019. There is strong needfor novel processes for the easy, effective preparation of large amountsof 14β-hydroxy-1,14-carbonate-baccatin III, and hence the derivativesthereof.

It has now been found that 14β-hydroxy-baccatin III-1,14-carbonate canbe prepared with a process starting from 13-ketobaccatin III, whichcompound can be easily obtained from 10-deacetylbaccatin III, which canin turn be easily isolated in large amounts from the leaves of Taxusbaccata, contrary to 14β-hydroxy-baccatin III.

Therefore, the invention relates to a process for the preparation of14β-hydroxy-baccatin III-1,14-carbonate which comprises the followingsteps:

a. treatment of 7-triethylsilyl-13-ketobaccatin III of formula

with suitable bases and oxidizing agents, to give7-triethylsilyl-13-keto-14-hydroxy-baccatin III:

b. carbonation of the 1 and 14 hydroxyls to give14β-Hydroxy-7-triethylsilyl-13-keto-baccatin III-1,14-carbonate:

c. reduction of the ketone at the 13-position and cleavage of theprotecting group in 7.

Starting 13-ketobaccatin III is conveniently protected at the 7-positionwith a suitable protective group, preferably selected from silyl ethers(preferably triethylsilyl ether). Step a) is carried out by treatmentwith a suitable base, in particular potassium t-butoxide (t-BuOK) orpotassium bis(trimethylsilyl)amide (KHMDS). The reaction can be carriedout at −40 to −78° C. Suitable solvents for this reaction are ethers,such as tetrahydrofuran or diethyl ether, in particular in mixture withhexamethylphosphoramide (HMPA) or1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)pyrimidinone (DMPU). The enolate isthen treated with an oxidizing agent, such as oxaziridine derivatives(in particular N-benzenesulfonyl phenyl oxaziridine, N-benzenesulfonylm-nitrophenyl oxaziridine and camphorsulfonyloxaziridine) to provide the7-protected 13-keto-14-hydroxy-baccatin III derivative.

Step b) is then carried out by treatment with a carbonylating agent (forexample carbonyldiimidazole or phosgene) under the conditions usuallydescribed in literature, to provide the 1,14-carbonate derivative. Thereaction can be conveniently carried out in inert solvents, preferablyethers or chlorinated solvents, in the presence of a base (preferablypyridine or triethylamine), at a temperature ranging from −40° C. toroom temperature. The reaction can be carried out both on the purestarting material and on the crude from the previous step.

The reduction of the carbonyl at the 13-position of step c) is easilycarried out with sodium borohydride in ethanol at a temperature usuallyranging from −20 to −50° C., and is completed within 2–6 hours. Thereaction can also be carried out in methanol, isopropanol, or in amethanol and tetrahydrofuran mixture. The reducing agent can be used instoichiometric amount, although an excess of hydride is preferably used.The reduction can also be effected with other hydrides, preferablytetrabutylammonium borohydride, lithium borohydride, sodium triacetoxyborohydride, in the conditions known in the art.

Protection at the 7-position is removed under conditions depending onthe protective group used. For example, if the protective group at the7-position is triethylsilyl ether, hydrolysis with hydrochloric acid inmethanol or hydrofluoric acid and pyridine in acetonitrile cansuccessfully be used.

13-Keto-baccatin III can conveniently be prepared by oxidation ofbaccatin III. Oxidation of baccatin III can be carried out with ozone,or with manganese dioxide in aprotic solvents such as methylenechloride, tetrahydrofuran, acetone, ethyl acetate. The reaction can becarried out at 0° C.–60° C., more preferably at room temperature.

The processes of the invention are summarized in the following scheme:

The following example further illustrates the invention.

The abbreviations used are as follows:

AcOEt=ethyl acetate; TES=triethylsilyl; TESCl=triethylsilyl chloride;DCM=dichloromethane, THF=tetrahydrofuran.

EXAMPLE

a) 13-Keto-baccatin III

Baccatin III (150 g, 0.25 mol) was dissolved in acetone (1.43 L).Commercially available manganese dioxide (450 g) was added in threeportions under strong stirring. After the starting product disappeared(4 h) the suspension was filtered and the solvent evaporated off. Thecrude was suspended in AcOEt (100 ml) and refluxed for 1 h, thenc-Hexane (100 ml) was added. The title compound was obtained from motherliquors, after evaporation of the solvent, as a white solid (140 g,95%).

b) 7-TES-13-keto-baccatin III

13-Keto-baccatin III (5 g, 8.5 mmol), TESCl (3.6mL, 21.4 mmol, 2.5 eq)and N-methylimidazole (2.73 mL, 34.3 mmol, 4 eq) were dissolved inanhydrous DCM (25 ml). The solution was left under stirring for 1.5 hthen quenched by slowly pouring it into 2M NaHSO₄ (25 ml). The aqueouslayer was washed, extracted with DCM (2×10 ml) and the combined organiclayers were extracted with brine (2×20 ml). The organic solution wasdried over sodium sulfate to give 4.7 g of the title compound,sufficiently pure for the subsequent step. M.p.: 212° C. TLC: cHex-AcOEt1:1, Rf=0.57. ¹H-NMR (200 MHz, CDCl₃) δ 0.58–0.66 (m, 6H, Si—CH₂);0.90–0.98 (t, J=8.4, 9H, CH₂CH₃); 1.21 (s, 3H, 17-Me); 1.27 (s, 3H,16,-Me); 1.69 (s, 3H, 19-Me); 1.83–1.96 (m, 1H, 6-H); 2.20 (s, 3H,18-Me); 2.21 (s, 3H, 10-OAc); 2.25 (s, 3H, 4-OAc); 2.48–2.65 (m, 1H,6-H); 2.81 (ABq, 2H, 14-H); 3.93 (d, J=6.6, 1H, 3-H); 4.25 (ABq, 2H,20-H); 4.51 (dd, J=10.6, 7.0, 1H, 7-H); 4.94 (d, J=7.7, 1H, 5-H); 5.72(d, J=7.0, 1H, 2-H); 6.61 (s, 1H, 10-H); 7.52 (t, J=6.2, 2H, Bz); 7.64(t, J=6.2, 11H, Bz); 8.10 (dd, J=7.4, 1.1, 2H, Bz).

c) 14-Hydroxy-7-TES-13-keto-baccatin III

7-TES-13-keto-baccatin III (670 mg, 0.96 mmol) was dissolved in amixture of anhydrous THF (9 ml) and DMPU (2 ml) and cooled to −60° C.under N₂. A 1M solution of t-BuOK in THF (2.5 ml, 0.86 mmol), previouslycooled to −50° C., was dropped therein. This solution was stirred at−60° C. for 45 minutes, then added drop by drop with(±)-camphorsulfonyl-oxaziridine (440 mg, 2 mmol) dissolved in anhydrousTHF (2 ml). The reaction mixture was stirred for 3 hours at −60° C. thenquenched with a 10% AcOH solution in anhydrous THF (2 ml). The mixturewas then left to warm at room temperature, then extracted with DCM (2×10ml). The combined organic layers were washed with water, a NaClsaturated aqueous solution of (15 ml) and dried over Na₂SO₄. The titlecompound was purified by flash chromatography (silica gel, cHex-AcOEt,8:2) in a 79% yield. Alternatively, this was used directly in thesubsequent step without further purification. M.p.: 94–98° C. TLC:cHex-AcOEt 1:1, Rf=0.5. ¹H-NMR (200 MHz, CDCl₃) δ 0.58–0.66 (m, 6H,Si—CH₂); 0.91–0.99 (t, J=8.7, 9H, CH₂CH₃); 1.24 (s, 3H, 17-Me); 1.28 (s,3H, 16,-Me); 1.75 (s, 3H, 19-Me); 1.83–2.05 (m, 1H, 6-H); 2.14 (s, 3H,18-Me); 2.24 (s, 3H, 10-OAc); 2.26 (s, 3H, 4-OAc); 2.46–2.61 (m, 1H,6-H); 3.64 (s, 1H, 1-OH) 3.73 (d, J=1.8, 1H, 14-OH); 3.87 (d, J=6.9, 1H,3-H); 4.14 (d, J=1.8, 1H, 14-H); 4.31 (s, 2H, 20-H); 4.49 (dd, J=10.7,6.6, 1H, 7-H); 4.93 (d, J=7.3, 1H, 5-H); 5.89 (d, J=7.0, 1H, 2-H); 6.53(s, 1H, 10-H); 7.46–7.66 (m, 3H, Bz); 8.08 (dd, J=7.0, 1.5, 2H, Bz)

d) 14β-Hydroxy-7-TES-13-keto-baccatin III 1,14-carbonate

A solution of 14β-hydroxy-7-TES-13-keto-baccatin (12.2 g) in anhydrousDCM (50 ml) and pyridine (16 ml) was dropped in a 20% phosgene solutionin DCM (45 mL, 5 eq) at −10° C. After 2 hours the reaction was addeddrop by drop with a 5% NaHCO₃ aqueous solution (100 ml). The aqueouslayer was washed with DCM (3×50 ml) and the crude was purified by flashchromatography (silica gel, DCM-AcOEt=50:1) to give the title compoundin a 95% yield. M.p.: 97–99° C. TLC: cHex-AcOEt 1:1, Rf=0.64. ¹H-NMR(200 MHz, CDCl₃) δ 0.58–0.66 (m, 6H, Si—CH₂); 0.91–0.99 (t, J=8.7, 9H,CH₂CH₃); 1.21 (s, 3H, 17-Me); 1.39 (s, 3H, 16,-Me); 1.75 (s, 3H, 19-Me);1.86–2.13 (m, 1H, 6-H); 2.22 (s, 3H, 18-Me); 2.25 (s, 3H, 10-OAc); 2.26(s, 3H, 4-OAc); 2.48–2.63 (m, 1H, 6-H); 3.83 (d, J=7.0, 1H, 3-H); 4.30(ABq, 2H, 20-H); 4.49 (dd, J=11.0, 7.0, 1H, 7-H); 4.81 (s, 1H, 14-H);4.93 (d, J=7.3, 1H, 5-H); 6.15 (d, J=7.0, 1H, 2-H); 6.54 (s, 1H, 10-H);7.51 (t, 2H, Bz); 7.62–7.70 (m, 1H, Bz); 8.01 (dd, J=7.0, 1.9, 2H, Bz).

e) 14β-Hydroxy-7-TES-baccatin III 1,14-carbonate

A suspension of NaBH₄ (0.5 g) in absolute ethanol (10 ml) was cooled to−50° C., and added with a cooled solution of14-hydroxy-7-TES-13-keto-baccatin III 1,14-β-carbonate (0.5 g, 0.6 mmol)in absolute ethanol (10 ml). After the starting product disappeared (8h), the reaction was quenched with citric acid and extracted with AcOEt.The combined organic layers were dried over sodium sulfate and thesolvent was evaporated off. The title compound was obtained as a whitesolid in an 85% yield, after chromatography. M.p.: 134–137° C. TLC:cHex-AcOEt 1:1, Rf=0.46. ¹H-NMR (200 MHz, CDCl₃) δ 0.58–0.66 (m, 6H,Si—CH₂); 0.91–0.99 (t, J=8.7, 9H, CH₂CH₃); 1.16 (s, 3H, 17-Me); 1.28 (s,3H, 16,-Me); 1.74 (s, 3H, 19-Me); 1.85–2.14 (m, 1H, 6-H); 2.06 (s, 3H,18-Me); 2.21 (s, 3H, 10-OAc); 2.33 (s, 3H, 4-OAc); 2.47–2.65 (m, 1H,6-H); 3.74 (d, J=7.4, 1H, 3-H); 4.12–4.35 (m, 2H, 20-H); 4.49 (dd,J=10.3, 6.6, 1H, 7-H); 4.82 (d, 1H, 14-H); 4.99 (d, J=7.3, 1H, 5-H);5.00–5.03 (m, 1H, 13-H); 6.11 (d, J=7.4, 1H, 2-H); 6.45 (s, 1H, 10-H);7.50 (t, 2H, Bz); 7.60–7.68 (m, 1H, Bz); 8.04 (dd, J=7.0, 1.5, 2H, Bz).

f) 14β-Hydroxy-baccatin III 1,14-carbonate

14-Hydroxy-7-TES-baccatin III 1,14-β-carbonate (9.6 g, 1.3 mmol) wasdissolved in a mixture of acetonitrile (5.4 ml) and pyridine (6.4 ml)cooled to 0° C. A solution of 70% HF in pyridine (0.95 ml) was droppedin 15 min and the solution was stirred at room temperature overnight.The reaction mixture was then poured into 20 mL of ice and left understirring for 1 h, then extracted with DCM (3×10 ml) and the combinedorganic layers were washed with 10% NaHSO₄ (to pH=2), 5% NaHCO₃ (2×10ml) and brine (2×10 ml). After evaporation of the solvent, the titlecompound was obtained as a white solid in a 96% yield.

1. A process for the preparation of 14β-hydroxy-1,14-carbonate-baccatinIII, which comprises: a. treatment of 7-triethylsilyl-13-ketobaccatinIII of formula

with suitable bases and oxidizing agents, to give7-triethylsilyl-13-keto-14-hydroxy-baccatin III:

b. carbonation of the 1 and 14 hydroxyls to give14β-Hydroxy-7-TES-13-keto-baccatin III-1,14-carbonate:

c. reduction of the ketone at the 13-position and cleavage of theprotective group in
 7. 2. The process as claimed in claim 1 wherein stepa) is carried out by treatment with potassium t-butoxide or potassiumbis(trimethylsilyl)amide at a temperature from −40 to −78° C. in ethersin admixture with hexamethylphosphoramide (HMPA) or1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)pyrimidinone (DMPU), in thepresence of oxaziridine derivatives.
 3. The process as claimed in claim2 wherein the oxaziridine derivative is selected from N-benzenesulfonylphenyl oxaziridine, N-benzenesulfonyl m-nitrophenyl oxaziridine andcamphorsulfonyloxaziridine.
 4. The process according to claim 1, whereinstep b) is carried out by treatment with a carbonyldiimidazole orphosgene in chlorinated solvents in the presence of a base attemperatures ranging from −40° C. to room temperature.
 5. The processaccording to claim 1, wherein step c) is carried out by treatment with ahydride at a temperature from −20 to −50° C.
 6. The process as claimedin claim 5 wherein the hydride is selected from sodium borohydride,lithium borohydride, sodium triacetoxy borohydride and the reaction iscarried out in ethanol, methanol, isopropanol, or in a methanol andtetrahydrofuran mixture.
 7. The process according to claim 1, wherein13-keto-baccatin III protected at the hydroxyl in 7 is prepared byselective acetylation of the hydroxyl 10 followed by oxidation of thehydroxyl 13 and protection of the hydroxyl
 7. 8. The process as claimedin claim 7 wherein 13-keto-baccatin III is obtained by selectiveacetylation of deacetylbaccatin III with acetic anhydride followed byoxidation with manganese dioxide in aprotic solvents at 0° C.–60° C. 9.A compound of formula:


10. A compound 14β-Hydroxy-7-TES-13-keto-baccatin III-1,14-carbonate, offormula:


11. The process according to claim 2, wherein step b) is carried out bytreatment with a carbonyldiimidazole or phosgene in chlorinated solventsin the presence of a base at temperatures ranging from −40° C. to roomtemperature.
 12. The process according to claim 3, wherein step b) iscarried out by treatment with a carbonyldiimidazole or phosgene inchlorinated solvents in the presence of a base at temperatures rangingfrom −40° C. to room temperature.
 13. The process according to claim 2,wherein step c) is carried out by treatment with a hydride at atemperature from −20 to −50° C.
 14. The process according to claim 3,wherein step c) is carried out by treatment with a hydride at atemperature from −20 to −50° C.
 15. The process according to claim 4,wherein step c) is carried out by treatment with a hydride at atemperature from −20 to −50° C.
 16. The process according to claim 2,wherein 13-keto-baccatin III protected at the hydroxyl in 7 is preparedby selective acetylation of the hydroxyl 10 followed by oxidation of thehydroxyl 13 and protection of the hydroxyl
 7. 17. The process accordingto claim 3, wherein 13-keto-baccatin III protected at the hydroxyl in 7is prepared by selective acetylation of the hydroxyl 10 followed byoxidation of the hydroxyl 13 and protection of the hydroxyl
 7. 18. Theprocess according to claim 4, wherein 13-keto-baccatin III protected atthe hydroxyl in 7 is prepared by selective acetylation of the hydroxyl10 followed by oxidation of the hydroxyl 13 and protection of thehydroxyl
 7. 19. The process according to claim 5, wherein13-keto-baccatin III protected at the hydroxyl in 7 is prepared byselective acetylation of the hydroxyl 10 followed by oxidation of thehydroxyl 13 and protection of the hydroxyl
 7. 20. The process accordingto claim 6, wherein 13-keto-baccatin III protected at the hydroxyl in 7is prepared by selective acetylation of the hydroxyl 10 followed byoxidation of the hydroxyl 13 and protection of the hydroxyl 7.